The type of rock that forms when material from dead plants and animals is deposited and hardens is called sedimentary rock.

Here's why:

* Sedimentary rocks are formed from the accumulation of sediments, which are small particles of rocks, minerals, and organic matter.

* Organic matter includes the remains of dead plants and animals.

* Over time, these sediments are buried under more layers of sediment, and the pressure and heat compact and cement the material together, forming rock.

There are different types of sedimentary rocks, and the one specifically formed from organic matter is called biogenic sedimentary rock, or biolith. Coal is a common example, formed from the accumulation of plant matter.